Your current antivirus program may offer plenty of protection, but new, unknown threats still could slip through. That's where PC Tools' ThreatFire comes in.
While you don't have to pay for PC Tools ThreatFire, installing it entails another kind of cost. The lightweight ThreatFire service and system tray process didn't noticeably impact our test system while we surfed the web and performed common tasks, and together they used less than 10MB of system memory. But security programs by nature have to reach far into your PC, and as a result they can often conflict with other software and cause trouble. In our tests on one heavily used PC with a wide range of installed programs and utilities, the computer locked up each time we attempted to run an on-demand scan using ThreatFire's signature scanner.
You don't need to run the signature scanner to get PC Tools ThreatFire's worthwhile behavioural protection, but such lockups are a good example of why you should be careful about installing too many security extras.